When configuring startup actions under the computer startup trigger, you find that these fail
Network access is required in order to copy files from a network share, we are only guaranteed to attempt actions when the network is available, for actions located under the 'Network Available' Trigger. Actions placed outside of 'Network Available' may be run 'before' the network is available and could fail.
By Default, all actions created under the Computer Trigger are run as 'SYSTEM'. You can select to run the action as a different user, one which has access to the network share in question.
A good troubleshooting step would be to enable auditing within your policy configuration for the specific trigger 'Network Available' to determine why the action failed (Events 9413 and 9414). It should confirm firstly, if the action was attempted and secondly, what the reason for the failure was such as 'Access Denied' or 'Path not found' etc